Nozzle
From Wikipedia, the free encyclopedia
A nozzle is a mechanical device designed to control the characteristics of a fluid flow as it exits from an enclosed chamber into some medium.
A nozzle is often a pipe or tube of varying cross sectional area, and it can be used to direct or modify the flow of a fluid (liquid or gas). Nozzles are frequently used to control the rate of flow, speed, direction, mass, and/or the pressure of the stream that emerges from them.
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[edit] Purposes of nozzles
[edit] High velocity nozzles
Frequently the goal is to increase the kinetic energy of the flowing medium at the expense of its pressure energy and/or internal energy.
Nozzles can be described as convergent (narrowing down from a wide diameter to a smaller diameter in the direction of the flow) or divergent (expanding from a smaller diameter to a larger one). A de Laval nozzle has a convergent section followed by a divergent section and is often called a convergent-divergent nozzle.
Convergent nozzles accelerate fluids. If the nozzle pressure ratio is high enough the flow will reach sonic velocity at the narrowest point (i.e. the nozzle throat). In this situation, the nozzle is said to be choked.
Increasing the nozzle pressure ratio further will not increase the throat Mach number beyond unity. Downstream (i.e. external to the nozzle) the flow is free to expand to supersonic velocities.
Divergent nozzles slow fluids, if the flow is subsonic, but accelerate sonic or supersonic fluids.
Convergent-divergent nozzles can therefore accelerate fluids that have choked in the convergent section to supersonic speeds. This CD process is more efficient than allowing a convergent nozzle to expand supersonically externally.
Since exhaust velocity has to exceed airspeed, supersonic aircraft also very typically use a con-di nozzle despite the weight and cost penalties. Supersonic jet engines, like those employed in fighters and SST aircraft (e.g. Concorde), indeed have relatively high nozzle pressure ratios. Because subsonic jet engines require low exhaust velocities, they require only subsonic exhaust and thus have modest nozzle pressure ratios and employ simple convergent nozzles.
Rocket motors use convergent-divergent nozzles, to maximise thrust and exhaust velocity and thus extremely high nozzle pressure ratios are employed.
Note that the Mach 1 can be a very high speed for a hot gas; since heat significantly raises the speed of sound. Thus the absolute speed reached at a nozzle throat can be far higher than the speed of sound at sea level. This fact is used extensively in rocketry where hypersonic flows are required.
[edit] Pipe Nozzles
Pipe nozzles are typically connections to tanks or pumps. They have characteristic entrance and exit pressure losses for fluid flow and have inherent structural features with respect to the tank, pump or equipment to which they are attached. Specific characteristics of entrance and exit losses can be found in the Crane "Flow of Fluids" Technical Paper No. 410 (often referred to as the Crane 410, see link: http://www.tp410.com/). Pipe nozzles may be flanged, butt weld end, or threaded connections. They typically have a load limit (force and stress) which is calculated in a finite element analysis program such as Caesar II or ANSYS.
[edit] Magnetic nozzles
Magnetic nozzles have also been proposed for some types of propulsion, in which the flow of plasma is directed by magnetic fields instead of walls made of solid matter.
[edit] Spray nozzles
Many nozzles atomise liquids. Often this involves Venturi tubes.
These kinds of nozzles are used for spray painting, perfumes, carburettors for internal combustion engines, spray on deodorants, antiperspirants and many other uses.
Air-Aspirating Nozzle-uses a opening in the cone shaped nozzle to inject air into a stream of water based foam (CAFS/AFFF/FFFP) to make the concentrate "foam up". Most commonly found on foam extinguishers and foam handlines.
[edit] Shaping nozzles
Some nozzles are shaped to produce a stream that is of a particular shape. For example Extrusion molding is a way of producing lengths of metals or plastics or other materials with a particular cross-section. These nozzles are typically referred to as a die.
[edit] Other meanings
In some areas of Scotland, the nozzle can refer to the nose.
It can also be used as an insult meaning something between 'jerk' and 'idiot'. This use was made popular by the character Al Calavicci on the science-fiction television show Quantum Leap.
